Field
The present invention relates to a light guide plate and a surface light source device, and more particularly, to a surface light source device used as a backlight of a liquid crystal display, and a light guide plate included in the surface light source device.
Related Art
The recent trend toward thinner mobile devices requires thinner surface light source devices incorporated in the mobile devices. Such a thinner surface light source device needs a thinner light guide plate. Although a light guide plate, which is a flat plate, can reduce its thickness, a light source such as an LED has a limitation in reducing its height. The thickness of the flat, thin light guide plate at its end face (light incident surface) can be greater than the height of the light source arranged to face the light incident surface of the light guide plate. In this case, the light source protrudes above the top surface of the light guide plate. Light emitted from the light source protruding above the light guide plate cannot entirely enter the light incident surface of the light guide plate and can partially leak out. This lowers the light use efficiency.
To overcome this disadvantage, a light guide plate including a flat light guide plate member may additionally include a light directing part thicker than the light guide plate member arranged at an end of the light guide plate member. The light directing part has an inclined surface that is inclined from its thickest position toward the end of the light guide plate member. Surface light source devices including such light guide plates are described in Patent Literature 1 and Patent Literature 2.
FIG. 1 shows an example of a surface light source device 11 incorporating a light guide plate including a light directing part thicker than a light guide plate member. The light guide plate 13 includes a light guide plate member 14 with a substantially uniform thickness, and a wedge-shaped light directing part 15. The light guide plate member 14 has a deflection pattern or a diffusion pattern on its back surface, and has a lenticular lens 16 arranged on its front surface. The light directing part 15 has an inclined surface 17, which is inclined from the thickest position of the light directing part 15 toward an end of the light guide plate member 14. The thickness of the light directing part 15 at its end face (light incident surface) is greater than the height of a light source 12. The surface light source device 11 including the light guide plate 13, in which the thickness of the light directing part 15 at its end face is greater than the height of the light source 12, allows light emitted from the light source 12 to efficiently enter the light directing part 15. The light entering the light directing part 15 is directed toward the light guide plate member 14 while it is widened into planar light. The light is reflected on the deflection pattern or the diffusion pattern and is emitted out of the light guide plate member 14 through its light emitting surface. The light emitted through the light emitting surface undergoes widening of the directivity with the lenticular lens 16. The surface light source device with this structure can be thin and can improve the light use efficiency of the light source.
In the surface light source device 11 including the light directing part 15 with the inclined surface 17, the light traveling obliquely with respect to the optical axis of the light source 12 in the light directing part 15 as viewed in the direction perpendicular to the light guide plate member 14 is reflected on the inclined surface 17 and widens in the lateral direction (in the width direction of the light guide plate). The resultant light forms a greater angle with the optical axis of the light source 12 as indicated by the arrows in FIG. 1. The light widened laterally on the inclined surface 17 can partially leak from the sides of the light guide plate 13, or can enter the lenticular lens 16 in the lateral direction and leak out of the lenticular lens 16. Such light loss would lower the light use efficiency and lower the uniformity of brightness.
A surface light source device described in Patent Literature 3 includes a directivity changing pattern arranged at one end of a light guide plate member near a light directing part. FIG. 2 is a plan view of a surface light source device 21 described in Patent Literature 3. The surface light source device 21 includes a light guide plate member 14, on which a lenticular lens 16 extends to a position inward from the end of the light guide plate member 14 to leave an area without the lenticular lens 16 between an inclined surface 17 of the light directing part 15 and the light guide plate member 14. In that area without the lenticular lens 16, a directivity changing pattern 22 is formed.
The directivity changing pattern 22 includes radially extending grooves. Each groove includes a downward inclined surface and an upward inclined surface. The downward inclined surface is inclined downward from the midpoint of the light guide plate toward the side. The upward inclined surface is inclined upward from the midpoint of the light guide plate toward the side. The downward inclined surface, which is inclined downward from the midpoint toward the side, has a greater width or a greater area than the upward inclined surface, which is inclined upward from the midpoint toward the side. As shown in FIG. 2, light entering the directivity changing pattern 22 and reflected by the directivity changing pattern 22 changes its directivity into a direction nearer the direction perpendicular to the light incident surface than the light that has yet to enter the directivity changing pattern 22. This structure reduces the amount of light leaking out of the surface light source device 21 from the sides of the light guide plate 13 or from the lenticular lens 16.
However, the surface light source device 21 described in Patent Literature 3 includes the light directing part 15 with a length K of, for example, 1.5 mm, and the directivity changing pattern 22 formed in the area with a length G of 1.5 mm. The surface light source device 21 thus has unused space, or space that is not used for emitting the illumination light, with at least a length of K+G=3.0 mm, which is the total length of the light directing part 15 and the area of the directivity changing pattern 22. This disables effective use of the area of the light guide plate 13.
Patent Literature 1: WO2010/070821
Patent Literature 2: Re-publication of PCT International Publication No. 2008-153024
Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2013-196799